Management of a process of freezing a frozen food product and the like involves an extremely important task of checking an internal temperature of this freezing target object. Thus, various measurement methods have conventionally been proposed. Such methods include: a method for inserting a temperature sensor in the form of a needle into the freezing target object and measuring the internal temperature; a method for measuring a surface temperature of the freezing target object by an infrared sensor, and a measurement method using change in electric capacity due to change in a physical state of water (see Non-Patent Literature 1 and Non-Patent Literature 2). Another proposed method uses electromagnetic waves in a microwave band to determine whether phase transition of the water content in the freezing target object has occurred (see Patent Literature 1).
When the temperature sensor in the form of a needle is used, the temperature sensor needs to be first inserted in the freezing target object. Thus, when the freezing target object is a solid food product, the inspected food product cannot be delivered to the market to be sold due to the issues of contamination with foreign matters and sanitation attributable to a needle hole remaining at the inserted position. All things considered, the temperature sensor in the form of a needle cannot achieve a 100% inspection of the internal temperature of food products in the process of freezing the food products.
The infrared sensor, which can measure the surface temperature of the freezing target object, cannot measure the internal temperature of the freezing target object, and cannot accurately measure the surface temperature of the food products with a packaging material such as a wrap. The method using the change in the electric capacity, described in Non-Patent Literatures 1 and 2, uses a special measurement device and involves detailed measurement conditions. Thus, the freezing target object is required to be arranged and measured under accurate conditions. Thus, this method is not suitably used for the 100% inspection of the temperature of the food products in the process of freezing the food products.
The method using the electromagnetic waves, described in Patent Literature 1, includes: irradiating an object containing water content with electromagnetic waves at a predetermined frequency emitted from a communication unit; receiving, with a wireless tag, electromagnetic waves, of the electromagnetic waves radiated, which have transmitted through the object; and emitting a response signal from the wireless tag that has received the electromagnetic waves transmitted to the communication unit. The levels of absorption and transmission of the electromagnetic waves in and through the water content largely vary among the phases of the water content that are a liquid phase, a solid phase, and a gaseous phase, that is, among water, ice, and vapor. Thus, the phases distinctively vary from each other in whether the communication unit can communicate with the wireless tag. All things considered, whether the phase transition of the water content has occurred can be determined based on whether the communication unit can communicate with the wireless tag.